I. Field of the Invention
This invention relates to a method of preparing a metal element adsorbent which can adsorb actinides such as uranium, thorium, transuranium elements, and the like; heavy metal elements such as cadmium, lead, chromium, mercury, iron, etc.; or metal elements such as cobalt, cesium, strontium, and the like.
Also, it relates to a method of adsorbing and separating the metal element from a solution containing the metal element by using the adsorbent.
II. Description of the Related Art
Nuclear fuel elements such as uranium, thorium, etc. are contained in the waste liquid effluent from nuclear fuel processing facilities. The prior art discloses a method of preparing an adsorbent for these nuclear fuel elements which is made from "kaki-shibu", that is, persimmon juice (Unexamined Published Japanese Patent Applications No. 63-61998 and No. 1-155947). This adsorbent is a hydrogel composition which is produced by reacting a kaki-shibu with an aldehyde or an acid such as sulfuric acid, phosphoric acid, etc., thereby causing the gelation of the persimmon tannin.
The present applicant has previously filed a patent application regarding a method for producing an adsorbent of insoluble tannin which comprises dissolving a tannin powder in an aqueous aldehyde solution, adding ammonia to the resulting solution to form a precipitate, and aging the precipitate to obtain the insoluble tannin (Unexamined Published Japanese Patent Application No.3-206094).
In the former method of preparing a hydrogel composition, the raw material is limited to kaki-shibu because tannins extracted from a natural product other than kaki-shibu will not react with an aldehyde or an acid, wherby a gel will not be formed.
The latter method of preparing an adsorbent of an insoluble tannin is problematic because the insoluble tannin is made from a tannin powder which is an aggregate of finely divided particles. When the insoluble tannin is packed into a column of a waste disposal plant and waste liquid is passed through the column, the insoluble tannin changes to the form of finely divided particles, thereby increasing the flow resistivity. For example, a waste liquid will pass through a column packed with an adsorbent consisting of the insoluble tannin at a space velocity of 17 h.sup.-1 at most. This rate means that it is insufficient to increase the throughput or processing ability of the waste liquid.